Cybercriminals are in the process of rebuilding the Gameover Zeus (GOZ) botnet, which law enforcement authorities took over in June, and recent research suggests that they’ve had some success, especially in the U.S.

The original GOZ botnet was built using a modified version of the infamous Zeus trojan program and was designed to steal online banking and other credentials from infected computers. The GOZ malware authors created a command-and-control infrastructure with a peer-to-peer architecture, making their botnet more resilient to takeover attempts.

Despite the technical challenges, the U.S. Department of Justice, working with foreign law enforcement agencies and private security companies, managed to seize control of the botnet in early June. Its size was estimated to be between 500,000 and 1 million infected computers at that time, with 25 percent of them located in the U.S.

On July 11, researchers from a company called Malcovery Security spotted a new variant of Gameover Zeus that had stopped using a peer-to-peer-based command-and-control infrastructure in favor of domain names.

Most malware programs are designed to connect to a hardcoded list of domain names associated with command-and-control servers. However, the new GOZ version uses a domain generation algorithm (DGA) to create a list of hundreds or thousands of new, random-looking domain names every day and then attempts to contact them.

Knowing how the DGA works, attackers can predict what domain names the malware will attempt to contact on a particular day. They can register one of them in advance, assign it to a server, and wait for the infected computers to connect in order to give them new instructions.

This makes it hard for security researchers to permanently take control of the botnet, because new domains will be generated constantly, but if they figure out the algorithm and register some of the domains, they can temporarily interact with it. This kind of operation, known as sinkholing, can at the very least be used to estimate the number of infected systems that are part of the botnet.

According to a report last week from researchers at Bitdefender, there are two new GOZ configurations using slightly different domain name generation algorithms—one that generates 1,000 domain names per day and one that generates 10,000. By sinkholing five domains over five different days for each of the two GOZ variants, the Bitdefender researchers counted 5,907 unique IP (Internet Protocol) addresses for computers infected by the first GOZ variant—almost 84 percent of them being from the U.S—and 4,316 IP addresses for the second variant, 70 percent of them from Ukraine and Belarus.

Counting IP addresses is not an accurate way of determining a botnet’s size because some computers receive a different IP address from their ISP every time they connect to the Internet. However, in the absence of better identifiers, it can at least be used as a rough estimate.

Security researchers from Arbor Networks have also sinkholed GOZ domain names in July, but have done it every four days in order to determine how the botnet evolves over time.

The company observed the number of victims gradually grow from 127 on July 14 to 429 on July 21. Then, on July 25, following a large spam campaign that distributed the new GOZ malware, the infection count jumped to 8,494 victims, many of them located in the U.S., the Arbor Networks researchers said Wednesday in a blog post.

“In aggregate and over three weeks, our five sinkholes saw 12,353 unique source IPs from all corners of the globe,” the researchers said. The most affected country was the United States, with 44 percent of infections, they said.

For now the creators of the new GOZ variant are focusing on rebuilding their botnet, rather than stealing money from users, but it’s likely only a matter of time until they’ll return to that primary goal.